The Catalyst::Engine sub-classes have all been removed and deprecated,
to be replaced with Plack handlers.

Plack is an implementation of the PSGI specification,
which is a standard interface between web servers and application frameworks.

This should be no different for developers,
and you should not have to migrate your applications unless you are using a custom engine already.

This change benefits Catalyst significantly by reducing the amount of code inside the framework,
and means that the framework gets upstream bug fixes in Plack,
and automatically gains support for any web server which a PSGI compliant handler is written for.

It also allows you more flexibility with your application,
and allows the use of cross web framework 'middleware'.

Developers are recommended to read Catalyst::Upgrading for notes about upgrading,
especially if you are using an unusual deployment method.

A number of modules have been updated to pass their tests or not produce deprecation warnings with the latest version of Catalyst.
It is recommended that you upgrade any of these that you are using after installing this version of Catalyst.

The development server restarter has been improved to be compatible with immutable Moose classes,
and also to optionally use B::Hooks::OP::Check::StashChange to handle more complex application layouts correctly.

Components are fully compatible with Moose,
and all Moose features,
such as method modifiers,
attributes,
roles,
BUILD and BUILDARGS methods are fully supported and may be used in components and applications.

Many reusable extensions which would previously have been plugins or base classes are better implemented as Moose roles.

There is a reasonable API in Catalyst::Controller for working with and registering actions,
allowing a controller sub-class to replace subroutine attributes for action declarations with an alternate syntax.